1. Field of the Invention
This invention is directed to improvements in impact resistant interlayer components for laminated glazing units. More particularly, the invention is directed to an alternative composite interlayer for the widely used interlayer material polyvinyl butyral in automotive windshields, sidelights, architectural glass and security glass, and to its method of manufacture.
2. Description of the Prior Art
Laminated glazing units, as employed in the windshields of automobiles, are generally produced by integrally bonding two sheets of glass to an interposed layer of plastic. Almost universally the plastic interlayer material employed is polyvinyl butyral. However, such resins are thermoplastic and have several drawbacks when used as an intermediate layer.
The surface of plasticized polyvinyl butyral film is very tacky and presents a problem of handling, often requiring storage and transport at low temperature. These difficulties arise primarily because of the presence of plasticizer in the polyvinyl butyral film. In practice, the degree and type of plasticization has to be carefully chosen so as to give the best compromise between ease of manufacture of the laminated glazing units and the properties of the laminated glazing unit after manufacture. Further, the moisture content of the plasticized polyvinyl butyral interlayer must be carefully controlled if satisfactory laminated glazing units are to be manufactured.
The difficulties associated with plasticized polyvinyl butyral and the public's increasing concern for a safer laminated glazing unit have le to many attempts over a long period of time to provide a more convenient and improved laminated glazing unit. For example, U.S. Pat. No. 4,427,743 to Katsuki et al shows a plastic sheet and a glass sheet bonded to each other through an adhesive layer of a thermoplastic resin containing no plasticizer. The adhesive layer minimizes the strain attributed to he difference in the coefficient of thermal expansion between the glass sheet and the plastic sheet. One possible adhesive mentioned is a copolymer of vinyl acetate and ethylene.
However, copolymers of ethylene and vinyl acetate suffer a loss of adhesiveness when applied directly to a glass substrate. Therefore, the laminated glazing units so produced do no possess sufficient retention of adhesiveness of the glass substrate to meet modern requirements for fragment retention. In addition, the impact resistance of the Katsuki et al glazing unit is derived from the thickness of the plastic sheet. However it is known as the thickness of the plastic sheet increases the rigidity and hence the safety of the glazing unit decreases.
U.S. Pat. No. 4,511,627 to Tanuma et al shows a sandwich glass comprising glass plates and an intermediate layer interposed therebetween. The intermediate layer is formed by crosslinking or photocrosslinking a resin composition comprising an ethylene vinyl acetate copolymer and an organic peroxide or a photosensitizer. However, the sandwich glass produced in accordance with the teaching of Tanuma et al does not possess the superior penetration resistance of the present invention.